The fact that the speed of sound is dependent on the medium through which the sound is passing can lead to quantitative inaccuracies in ultrasound image capture (e.g., in image capture for the human body). While structures reproduced in an ultrasound image may be displaced correctly in terms of their shape, an actual distance of actual distances from an ultrasound head to the structures themselves may not be correctly shown in the reproduced ultrasound image.
Correcting for speed-of-sound factors (the effect of different speeds of sound in different penetrated media) is less important in the field of medical diagnostics, as the exact or absolute position of an object relative to the ultrasound head is not of great importance. As a result, only a few attempts have been made to determine or correct speed-of-sound factors or to correct geometric distortions and aberration errors caused by speed-of-sound factors.
An attempt to directly register ultrasound images, wherein only the ultrasound images themselves are used (e.g., two sound images in which corresponding structures are sought) is know from an article by Kruücker et al. entitled “Sound Speed Estimation Using Automatic Ultrasound Registration”, from IEEE Transaction on Ultrasonic Ferroelectrics and Frequency Control, 51(9): 1095 to 1106, 2004. These attempts, however, suffer greatly from significant distortions in the captured image data, from a large number of artefacts, problems with collecting data due to scattering effects, and numerical difficulties.
Another approach for taking into account or correcting different speed-of-sound factors, proposed in Barratt et al. “Self-Calibrating Ultrasound-to-CT Bone Registration” from MICCAI (Conference Proceedings), LNCS 3749, Springer Publishing, pages 605 to 612, is to additionally optimize scaling parameters during three-dimensional registration of ultrasound data sets, for example onto a CT data set. In this method the parameters relevant to the essential recording geometry of the ultrasound imaging process may be integrated into the registration method. Since in this method, optimization treats all ultrasound tomographs equally, the scaling parameters are adapted overall. Thus, the speed-of-sound factors only can be estimated or corrected overall.
Speed-of-sound factors may be locally estimated to some extent in methods for time-of-flight ultrasound tomography. This has been performed, for example, in Glover, Sharp: “Reconstruction of Ultrasound Propagation Speed Distributions in Soft Tissue: Time-of-Flight Tomography” from IEEE Transactions on Sonics and Ultrasonics, Volume 24, No. 4, pages 299 to 234, 1977. However, due to technical limitations, such as, for example, noise, geometric distortions, low sound penetration and other artefacts, as well as inherent mathematical properties, in particular the numerical instability of the mathematics to be solved for reconstructing the speed-of-sound factors, these approaches are associated with significant difficulties that severely restrict the practical use of the methods for diagnostic purposes as well as for assisting medical navigation methods. The data ascertained in this manner are not reliable for precisely estimating speed-of-sound factors.